Abstract

Magnetomotive optical coherence tomography (MM-OCT) is a functional extension of OCT which utilizes magnetically responsive materials that are modulated by an external magnetic field for contrast enhancement and for elastography to assess the structural and viscoelastic properties of the surrounding tissues. Traditionally, magnetomotive contrast relies on the interaction between the displacement of magnetic particles induced by an external magnetic field and the micro-environmental restoring (elastic) force acting on the particles. When the restoring force from a sample containing magnetic particles is weak or non-existent, the MM-OCT signal-to-noise ratio (SNR) can degrade significantly. We have developed a novel solenoid configuration to enable MM-OCT imaging in samples that do not have an elastic restoring force, such as liquids. This coil configuration may potentially enable real-time MM-OCT imaging.

Figures (6)

Schematic and output of the dual-coil MM-OCT setup: A) Dual-coil MM-OCT system. The scanning beam for OCT imaging passes through the bore at the center of the coil. B) Visualization of the time-dependent dual-coil activation and the resulting MNP displacement. C) Peak AC magnetic field strength (Gauss) at different modulation frequencies.

Structure of the magnetic protein-shell, oil-core, microspheres: A) Conceptual drawing of a magnetic microsphere with core-shell structure. B and C) Transmission electron microscopy (TEM) images of a single microsphere showing the presence of magnetic nanoparticles and aggregates within the oil-core. The region in the dashed-yellow box in (B) is magnified and shown in (C).

Magnetomotive OCT images and signal with a dual-coil configuration: A, B, and C) OCT B-mode images of two plastic tubes (bare microspheres (MSs) as a control (left) and magnetic microspheres (right)). D, E, F, G, H, and I) Magnified regions from the corresponding colored boxes in (A, B, and C). Included videos (Media 1 and Media 2) are the OCT images corresponding to (G and I). J, K, L, M, N, and O) Spectral analysis extracted from along the corresponding red lines in (D, E, F, G, H, and I). The spectral analysis results (M and O) revealed the frequency component identical to the external AC magnetic field frequency (100 Hz and 1 kHz), which appeared only when the AC magnetic field was ON.

Representative MM-OCT images of flowing MSs captured under flow (Media 4): A) When the external magnetic field is OFF. B) Power spectrum of two MSs (red and green lines) in (A). C) When the external magnetic field of 200 Hz is ON. D) Power spectrum of two MSs (red and green lines) in (C). E) When the external magnetic field of 1 kHz is ON. F) Power spectrum of two MSs (red and green lines) in (E). The external AC magnetic field frequency (200 Hz and 1 kHz) is clearly visible in the frequency spectrum analysis (D and F; red line). The yellow and orange arrows in (A and B) indicate flow direction and magnetic MSs, respectively. The pink arrows in (C and E) indicate the modulating magnetic MSs.

Comparison of M-mode MM-OCT imaging for A) single-coil configuration and B) dual-coil configuration. The orange arrows indicate MSs modulated by the external magnetic field. The MSs are drifting upward over time when using a single-coil (A), and oscillate sinusoidally with zero-offset when using the dual-coil configuration (B).